Re: As was common back then ...
Better late than never ;-)
Personally I've never, ever heard "gwyn" used to mean "fat", unless perhaps it's in a phrase such as "the white of the meat", which could refer to fat on a joint I suppose. The usual word for "fat" as in "he's fat" is "tew" ("tehoo" - said as one syllable), hence the Welsh for hamster is "bochdew", i.e. "fat cheeks".
The usual word for fat, meaning the fat on meat, is "braster" (or brasder), "saim" ("saheem" - one syllable again) is used for "grease". Sorry about my transliterations; it's difficult!
There may well be some very local dialect which has different words, but this is what I heard in school (South East Wales) and what I hear at work (South West Wales).